Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $10.74   12.564%   $12.0894   $12.09   $12.10   $12.10  
  Purchase #2 $22.30   3.770%   $23.1407   $23.14   $23.15   $23.10  
  Purchase #3 $195.96   13.187%   $221.8012   $221.80   $221.80   $221.80  
  Purchase #4 $116.39   9.847%   $127.8509   $127.85   $127.85   $127.90  
  Purchase #5 $101.44   5.500%   $107.0192   $107.02   $107.00   $107.00  
  Purchase #6 $18.89   6.817%   $20.1777   $20.18   $20.20   $20.20  
  Purchase #7 $171.71   10.823%   $190.2942   $190.29   $190.30   $190.30  
  7 purchase totals = $637.43       $702.3733   $702.37   $702.40   $702.40